"The appearance of St. Elmo's Fire was regarded as a good omen to sailors, it tended to occur in the dissipating stages of severe thunderstorms when the most violent surface winds and seas were abating. Thus, it was interpreted as the answer to the sailors' prayers for heavenly intervention as their patron saint Elmo had come to watch over them and see them safely through the storm."
I've seen 2 really good displays in the last year whilst flying B777s through the ITCZ in Africa. On one occasion we avoided the green returns on our radar by about 40nm which fortunately resulted in a smooth ride. However, we still had the most fantastic display of St. Elmo's Fire on the front windscreens. It lasted about 5 minutes with constant flashes of blue branch-like discharges at its most active. Unfortunately, I didn't have a camera with me.
I love watching St. Elmo's Fire and we often invite the cabin crew up to the flight deck to watch these displays.